A novel cavity-first method was adapted to the fabrication process of a planar vibration sensor. First, the proof mass was under etched, after which the etch holes that were created in the proof mass in order to release them by under etching were filled with a fluoropolymer. After this planarization step, metal interconnects were patterned. By using the novel cavity-first micromachining process, a planar mode piezoresistive vibration sensor was successfully fabricated with both a simple structure and fewer packaging difficulties. Several significant case studies of random vibrations, TTH signal waves, and earthquake waves were evaluated comprehensively. We also measured the output noise spectral density of the fabricated sensor. In doing so, one of most important mechanical properties of the given planar sensor can be understood. By comparing the response results, the planar sensor exhibited performance that was comparable to that of a high-quality commercial device.